1. Field of the Invention
The present invention generally relates to surface acoustic wave filters and duplexers using the same, and more particularly to a surface acoustic wave filter in which one IDT (InterDigital Transducer) is divided in multiple IDTs connected in series and a duplexer using such a filter.
2. Description of the Related Art
Recently, a filter of a surface acoustic wave device has been employed in the RF circuit of radio equipment such as a portable phone. The filter of the surface acoustic wave device (hereinafter referred to as SAW filter) may be a transmit filter, a receiver filter or an antenna duplexer equipped with the transmit filter and the receive filter that are packaged as a single device.
FIG. 1 is a block diagram of a conventional antenna duplexer used in a portable phone. A transmit signal applied via a transmit terminal 13 passes through a transmit filter 16, and is output via an antenna terminal 14. A receive signal received via the antenna terminal 14 passes through a matching circuit 12 and a receive filter 11, and is output via a receive terminal 15.
Antenna duplexers are disclosed in FIG. 6 of Japanese Patent Application Publication No. 2003-249842 (hereinafter referred to as Document 1) and FIG. 30 of Japanese Patent Application Publication No. 2004-194269 (hereinafter referred to as Document 2). These antenna duplexers have a transmit filter formed by a ladder type SAW filter, and a receive filter formed by a multimode type SAW filter. The transmit filter may receive high power and is therefore formed by the SAW filter having high power durability. The receive filter is required to have a high out-of-band attenuation and a steep cutoff characteristic, and is therefore formed by the multimode type SAW filter.
The basic structure of the multimode type SAW filter is disclosed in, for example, FIG. 5 of Document 2, and is composed of a pair of reflectors formed on a piezoelectric substrate, an input IDT (composed of comb-like electrodes) and an output IDT. The input and output IDTs are interposed between the pair of reflectors. A drive voltage is applied to the input IDT, and resultant SAWs are propagated between the reflectors. There are multiple standing waves between the reflectors. Voltages that correspond to the frequencies of the standing waves appear at the output IDT. The multimode type SAW filter functions as a bandpass filter.
However, the duplexers disclosed in Documents 1 and 2 have the following problems. Referring to FIG. 1, input power of the transmit signal applied via the transmit terminal 13 passes through the transmit filter 16 and reaches the antenna terminal 14, as indicated by the solid-line arrow. However, as indicated by a dotted-line arrow, some input power passes through the matching circuit 12 and reaches the receive filter 11 as leakage power. The multimode type SAW filter has poor power durability, and the leakage power may break the receive filter or may cause non-linearity of the receive filter 11. This degrades the receive sensitivity.